1. Field of the Invention
The present invention relates generally to nuclear medical imaging devices and more particularly relates to calibration of scintillation cameras to enable correction of acquired image data for unavoidable distortions caused by the inherent physical characteristics of the detector head of the scintillation camera, such as crystal thickness and photomultiplier tube parameters.
2. Introduction
In various environments, such as in medical environments, imaging devices can include detectors that detect electromagnetic radiation emitted from radioactive isotopes or the like within a patient. The detectors typically include a sheet of scintillation crystal material that interacts with gamma rays emitted by the isotope to produce photons in the visible light spectrum known as “events.” The scintillation camera includes one or more photodetectors such as an array of photomultiplier tubes, which detect the intensity and location of the events and accumulate this data to acquire clinically significant images that are rendered on a computer display for analysis.
Existing scintillation cameras experience spatial distortion that requires linearity correction (LC). The spatial distortion arises from the fact that the spatial coordinates of light events occurring either at the edges of or between adjacent photomultiplier tubes in a photodetector array will be computed differently than the coordinates of events occurring directly over the center of a photomultiplier tube, due to the physical limitations of the photomultiplier tube. A significant amount of effort has been made to developing correction schemes for spatial or linearity distortion (along with, e.g., the companion energy and flood corrections). Existing LC methods can be generally divided into two categories.
A first category is illustrated in U.S. Pat. No. 3,745,345 (the '345 patent) entitled Radiation Imaging Device, the entire disclosure of which is incorporated herein by reference. Here, a camera head is covered by a lead mask having a uniform grid of pinhole apertures. A sheet source of uniform radiation placed adjacent to the mask causes each aperture to illuminate a scintillation crystal located on the opposite side of the mask. The camera then records the detected location of events in the crystal. There is a difference between the (known) location of the pinholes and the detected location of the events as computed by the camera, which is representative of the degree of spatial distortion at the respective locations on the camera face. Accordingly, a correction factor is computed for each location point so as to move the apparent location of an event as detected to its actual location, as determined by the difference computed in the flood source calibration procedure. The correction factors are then stored in an array for later use during acquisition of clinical images.
A second category is illustrated in U.S. Pat. No. 4,212,061 entitled Radiation Signal Processing and U.S. Pat. No. 4,316,257 entitled Dynamic Modification Of Spatial Distortion Correction Capabilities Of Scintillation Camera, which pertain to spatial correction (both the '061 and '257 patents also are incorporated herein in their entirety by reference). For calibration, a lead mask having elongated slit apertures is used. The camera is exposed to a radiation source, first with the mask oriented in x lines and then with the mask oriented in y lines. For each such exposure orientation, a series of transverse peak measurements at select intervals is developed. An analytical expression is generated to represent event coordinates between calibration intervals. Each orientation exposure, thus, produces one of a pair of calibration coordinates, which in turn permit direct correspondence to associated spatial coordinates. Among other deficiencies in this method, this method can take more than one hour of time by itself. It also requires additional preparation such as ‘centering and gain’. Moreover, this method requires use of multiple masks wastes time and money and increases equipment downtime.
Although there has been a significant amount of effort applied in the development of procedures for LC, the lead masks used in the processes have received little attention. The flood masks utilized in prior art devices have involved pinhole apertures arranged in a uniform and rectangular distribution (such as depicted in FIGS. 1 and 2 of the '345 patent). This design has a number of deficiencies, such as: a) generating a relatively low number of data points; and b) being less reliable where spatial distortion is more severe, such as near edges of photomultiplier tubes and/or when thicker scintillation crystals are employed. In addition, existing lead masks do not enhance functionality in the overall calibration process, such as to enable shorter calibration times and/or higher accuracies.
The present inventors have co-developed a new type of flood calibration mask having a much denser population of pinhole apertures in a non-uniform grid pattern, which is used in conjunction with a novel Gaussian fit algorithm to obtain a complete pinhole mask image model for LC coefficient generation. A LC coefficient represents a displacement vector of a point from its detected location in an acquired image to an ideal location.
An important step of the LC coefficient generation process is the accurate determination of spatial coordinates of the pinhole positions in the image. The present inventors have discovered that, while an initial estimation of the pinhole coordinates may be derived in an a priori fashion for a particular type of scintillation crystal and PMT configuration, the use of an a priori model pinhole location algorithm achieves less than optimal accuracy over a large distribution of gamma cameras, because of the large differences in physical parameters from camera to camera.
There thus remains a need for improvements in pinhole position estimation so that the a priori model can be accurately aligned to the acquired image on a camera-by-camera basis over a wide distribution of camera physical parameters.